A commonly performed orthognathic surgical procedure is the SSRO (sagittal split ramus osteotomy). This procedure is typically performed on patients with a malocclusion or condition in which the lower teeth of the mandible do not line up with the upper teeth of the maxilla. These conditions are most commonly manifest in what are known as an "overbite" or an "underbite."
The SSRO surgical procedure involves performing osteotomies or bone cuts in a specific pattern in the buccal (towards the cheek) and lingual (towards the tongue) cortical plates of the mandible such that the cortical plates are split and the mandible is divided into two proximal segments bilaterally and a distal segment centrally. The distal or tooth bearing fragment is thus freed from the proximal, condylar bearing fragments, which form the joints with the base of the skull, such that the distal fragment can be positioned into the correct occlusal relationship with the teeth of the maxilla.
The teeth of the distal fragment, and hence the distal fragment itself, are typically fixed by wires to the teeth of the maxilla during the surgical procedure to establish its correct positioning. Once this has been accomplished, the surgeon is faced with the task of repositioning the free proximal segments prior to application of fixation hardware to reunite the proximal and distal fragments for bone healing in their new relative positions.
The most important consideration in repositioning the proximal segments prior to application of the fixation hardware is to insure that the condylar part of the proximal segment is correctly seated in the glenoid fossa of the base of the skull. This is of critical importance for a number of reasons. If the condyle is not correctly seated into the glenoid fossa at the time of application of the fixation hardware a malocclusion will occur post-operatively. This malocclusion can be caused by the pull of muscles attached to the mandible that will work to reseat the condyle into the fossa to create a functional joint condition. Since the distal segment has been reattached to the proximal segments via the fixation hardware and later by healing of the bone, the condyles migration back into the fossa will move the mandibular teeth back into a condition of malocclusion, negating the whole intent of the surgery in the first place.
There is also the possibility that if the condyle is not correctly positioned and seated in the fossa at the time of surgery, it may not become fully reseated in the fossa by muscle forces post-surgically.
Surgeons have recognized the importance of correctly positioning the condyle in the fossa during the SSRO procedure and have developed a number of techniques and appliances to accomplish this seating. The appliances have traditionally taken the form of a bone fixation plate that is attached to the maxilla and the proximal segment prior to separation of the proximal and distal mandibular segments.
The intent of these devices is to maintain the position of the proximal segment relative to the fossa throughout the surgical procedure and application of fixation hardware. Once the fixation hardware is applied, the positioning appliance can be removed.
A key problem with this approach involves the fact that the patient is under anesthesia prior to application of these appliances. Because of a lack of muscle tonus during anesthesia, the condyle "falls" out of the fossa and is therefore not in fact correctly positioned at all.
Another common technique for seating of the condyle involves the use of a gauze packer instrument. The gauze packer is an instrument with a coaxial handle and shaft with a bifurcated or forked end. After splitting the proximal segments from the distal segment, the surgeon uses a rotating cutting burr to create a notch in the front edge of the proximal segment. The surgeon then uses the gauze packer instrument by placing its forked end into the notch and directing a force via the instrument into the proximal segment posteriorly and inferiorly while at the same time lifting up externally at the angle of the mandible with digital pressure. The resultant forces applied to the proximal segment serve to push the condyle upwards and forward into the fossa to effect the seating of the condyle. This technique is known as the bi-vector condylar seating technique. The fixation hardware is then applied to fix the segments in their new relative positions.
This instrument has the drawback of only allowing the surgeon to apply forces to the proximal segment in basically one direction. This is particularly disadvantageous at the beginning of the condylar seating process when the surgeon may wish to manipulate the proximal segment in a number of different directions and planes to get the segment settled into an appropriate position relative to the soft tissues and the distal segment prior to final seating.